Positive acting resist media sensitive to radiation, e.g. electron beam, and the use thereof in forming surface patterns in a variety of substrates are well known in the art. Poly(olefin sulfone) polymers are a major class of such positive resist media. For example, poly(1-methyl-1-cyclopentene sulfone) has been utilized for audio/video recording apparatus as described in Poliniak et al. U.S. Pat. Nos. 3,935,331 and 3,935,332. Poly(1-butene sulfone) has been employed for integrated circuit mask formation and poly(cyclopentene sulfone) and poly(bicycloheptene sulfone) have been employed for multilayer integrated circuit mask formation.
In general, surface relief patterns are formed in a layer of a poly(olefin sulfone) or poly(acetylene sulfone) copolymer by dissolving the polymer in a suitable solvent, forming a wet layer of the resulting solution on a substrate such as a metal, baking or vacuum drying the wet layer to remove the solvent, irradiating the dry layer with e.g. a modulated beam of electrons, and developing the copolymer layer with a solvent to yield the desired surface relief pattern. To be useful as a positive resist media, such a film must degrade upon irradiation, be developable in the exposed areas with good resolution, and be resistant to chemical etching solutions. Non-irradiated areas of the film must adhere well to the substrate and be resistant to cracking and crazing.
The choice of a specific polymer for use as a radiation sensitive resist media heretofore has necessitated a balancing of such properties. For example, in order to prepare a resist film having a high degree of sensitivity, one generally will select a polymeric material having high molecular weight and low molecular weight distribution. These parameters, however, usually contribute to polymer brittleness. The converse is often true with low molecular weight polymers. Other equally important criteria, such as retention of resist sensitivity and fine line resolution, must be balanced as well in selecting a polymeric material.
For example, a positive resist coating of poly(3-methyl-cyclopentene sulfone), such as is described in E. S. Poliniak et al., U.S. Pat. No. 4,153,741, when spun on a chromium substrate to a thickness greater than about 3000 angstroms may show varying degrees of cracking or crazing upon development, depending on the type and quality of the chrome substrate, the coating and baking procedures, and the characteristics of the resist material itself. The addition of a conventional plasticizer will eliminate cracking in such coatings up to a thickness of about 6000 angstroms. However, there is a concomitant deterioration in fine line resolution and resist sensitivity.
Poly(olefin sulfone) terpolymers and their use as radiation sensitive resist recording media are likewise known in the art. For example, Gipstein et al. in U.S. Pat. No. 3,898,350 disclose terpolymers formed from an alpha olefin, sulfur dioxide, and a compound selected from the group consisting of cyclopentene, bicycloheptene and methyl methacrylate.
Gipstein et al. indicate that poly(olefin sulfone) terpolymers such as are disclosed therein have lower glass transition temperatures and are, therefore, less brittle than copolymers made from the disclosed olefins, e.g. cyclopentene and bicycloheptene. The disclosed terpolymers are indicated as being resistant to cracking in films up to 9100 angstroms thick. In contrast, sulfur dioxide copolymers of the disclosed group of monomers represented by cyclopentene and bicycloheptene exhibit cracking in film thicknesses greater than about 3000 angstroms. The terpolymers disclosed by Gipstein et al. are of little practical value, however, since their sensitivities appear to be above the 1 .mu.C/cm.sup.2 dose level generally recognized as an upper limit for a commercial high throughput radiation sensitive resist.
The present invention is predicated on the discovery that small amounts of an omega alkynoic acid as a termonomer in a sulfone terpolymer significantly improve certain desirable characteristics of radiation sensitive resist media formed therefrom without loss in sensitivity.